Electronic sensors and related control devices are widely used in the field of industrial controls. For example, photoelectric sensors are used for detecting the presence or absence of an object, and for providing a signal or switching accordingly.
Most such sensor and control devices which are intended to operate on low voltage DC provide either a current sourcing output or a current sinking output. The current sourcing output is typically a PNP transistor and the current sinking output is typically a NPN transistor. European engineers have traditionally preferred the current sourcing outputs, while American and Japanese engineers have traditionally preferred current sinking outputs. Due to the global aspects of today's markets, it is desirable to sell sensor and control devices with both types of outputs. However, selling different models, each of a different output type, doubles the inventory requirements for the seller.
Some companies manufacture such devices with two different output wires: one wire for current sourcing output and one wire for current sinking output. This is a good solution, except that it requires an extra lead on output. Typically, only one or the other type of output is used by the customer. Since many customers request a connector with the device, the extra lead results in an extra pin on the connector. Furthermore, some devices have more than one output function. If each output is to have a sourcing and sinking output, the need for extra leads increases.
One solution is to install a switch in the device to allow selection of the output type. This is undesirable due to the resulting size increase in the device. An additional problem is that switches are not as reliable as solid state components.
Another solution is to supply a "push-pull" type output for the device. In this type output, a sourcing transistor output and a sinking transistor output are tied together and driven such that only one of the transistors conducts at a given time. Such a configuration eliminates the need for the extra lead, but it makes it impossible to tie outputs of multiple devices together in parallel to provide the "wired OR" function. If multiple "push-pull" type outputs were tied together in parallel, there would be a short circuit across the supply line if one output was sourcing and the other sinking.
The present invention provides improvements in the design of sensors and control devices of the type described above through the use of a "push-pull" type output circuit that disables one or the other of the output transistors.